Qualitative Analysis Essays - Chemistry, Silver Compounds

Qualitative Analysis Qualitative analysis is used in the determination of the identity of a substance. It is different from quantitative analysis, which deals with the determination of the amount of a substance. In this experiment, qualitative analysis techniques are used to determine whether or not a sample contains a certain ion. When using this method, an unknown and a reactant are mixed. The result of the reaction leads to a conclusion about the presence or absence of certain ions in the unknown. Many ions react in similar ways, and although the addition of one reagent to an unknown may not identify the ion, it limits the possibilities as to what the ion could be. A sequence of reactions used to analyze a sample is called a scheme, and it usually requires a large number of reagents and separation steps. For this experiment, the unknown may contain anywhere from 2 to all of the following cations and anions: Cations Anions Ag+ Cl- Ba2+ SO42- Fe3+ PO43- Cu2+ Cr3+ The following reagents are used to identify the ions: 1M H2SO4 2M HCl 2M NH4OH (labeled as NH4+) 2M NaOH .1M Ba(NO3)2 (labeled as .1M Ba2+) .1M AgNO3 (labeled as .1M Ag+) The first four are used to identify the cations, and the last two, used in conjunction with the first four, are used to identify the anions. The identification of the ions is mainly based on solubilities. This means that something must be known about the solubility characteristics of the different ions in the presence of the available reagents. The point of the first part of the experiment is to learn which reagents cause the ions to form precipitates, and which reagents dissolve the precipitates formed by the ions. This information is used to make the flow charts for the identification on the unknown ions. For example, it is important to know that a certain reagent will dissolve the precipitate formed by one ion, while it will not dissolve the precipitate formed by another ion. This can be used to distinguish between two different precipitates present in a solution, or to confirm which ion formed the precipitate and therefore was present in the solution. When carrying out the reactions, avoid adding an excess of reagent to the solution. This is because some precipitates redissolve in an excess of the reagent. Therefore, in cases where one drop of reagent produces a precipitate, 3 or more drops could completely dissolve the precipitate without it ever being visible to the eye. This would cause a large error in the scheme developed to identify the unknown ions. Experimental: The first part of the experiment consists of reacting the cations and anions with the reagents in order to see what the reaction will result in (precipitate or no precipitate). The cations were each reacted with the first four reagents listed in the introduction (H2SO4, HCl, NH4+, and NaOH). Then, the anions were each reacted with Ba2+ and Ag+. This was done by placing 2 drops of the ion in the test tube and then adding 2 drops of reagent. Each cation was reacted with each of the 4 reagents before moving on to the next cation to be tested. Prior to performing the reactions, a chart was made like the one in the data and calculations section. As each reaction was performed, the chart was filled in with the observation of what happened. If there was no change, NR was written in the chart for "no reaction." If a precipitate formed, the color of the precipitate was written in the chart. If there was no precipitate but there was a color change in the solution, that was also recorded. As each reaction was carried out, it was sometimes difficult to determine whether a precipitate formed or not. If there was uncertainty, the test tubes had to be placed into the centrifuge in order to separate the precipitates from the solution. There are some very important things to remember when using the centrifuge. First, when tubes are placed in the centrifuge, a tube with an approximately equal volume of solution should be placed exactly opposite each sample tube to counterbalance it (use a test tube filled with an equivalent amount of water if necessary). Second, the centrifuge should come to a stop before it is opened and the test tubes removed. This is to avoid injury. Once the tubes were removed from the centrifuge, it was obvious whether there was a precipitate present or not. If a solid has settled onto the bottom or side of